convert sql database to oracle

You need to check these :
Migrating a Microsoft SQL Server Database to Oracle Database 11g[^]
How to Convert SQL server to Oracle?[^]
Migrating Oracle Databases to SQL Server[^]

Yes, possible but it's a big task.

If you go with 3rd party tools then it's not that very big job.
Go with Oracle[^]. You could find more 3rd party tools related to these type conversion/migration, search google. I'm not sure you'll get any FREE or open source tool for this task.

If you can't afford for 3rd party tools, then you'll have to start a big process like below one.
Microsoft SQL Server and Oracle Compared[^]
Based on the comparison, you have to do many changes in process. That's it.